Since its beginnings in the late 1940's, the offshore petroleum industry has been moving into progressively deeper waters. Until recently, offshore petroleum drilling and producing operations typically have been conducted from rigid, bottom-founded offshore structures such as conventional steel jacket structures or concrete or steel gravity structures. Such structures are designed to rigidly resist environmental forces such as wind, waves, and ocean currents. However, in waters deeper than about 1,000 feet, the steel tonnage, and hence the cost, required to rigidly resist environmental forces increases so rapidly that an economic limit is soon reached, even given the most favorable economic conditions.
Water depths of interest to the offshore petroleum industry have now increased to the point where rigid, bottom-founded offshore structures are, in many cases, no longer technically or economically feasible. This problem has resulted in the development of new types of offshore structures generally known as "compliant towers". Compliant towers are offshore structures that do not rigidly resist environmental forces. Rather a compliant tower is designed to yield to the environmental forces in a controlled manner. Basically, the tower is allowed to oscillate a few degrees from vertical about its base in response to the applied environmental forces. This oscillation, which may be characterized as that of an inverted pendulum, creates an inertial restoring force which opposes the applied environmental forces.
One such compliant tower is the "guyed tower". Basically, a guyed tower is a trussed structure of generally uniform cross-section that extends upwardly from the bottom of the body of water to a deck supported above the water surface. The structure is held upright by an array of guy lines which are spaced around the periphery of the structure and radiate outwardly and downwardly to anchor points located on the bottom of the body of water. The guy lines permit the tower to pivot laterally a few degrees about its base in response to surface wind, wave, or current forces. See generally, Finn, L. D., "A New Deep-Water Platform--The Guyed Tower", Journal of Petroleum Technology, April 1978, pp. 537-544.
A second type of compliant tower is the "buoyant tower". Basically, a buoyant tower is a trussed structure similar to a guyed tower; however, no guy lines are used. The entire restoring force for the structure is provided by large buoyancy tanks located on the structure, preferably at or near the surface of the body of water. See, for example, the buoyant tower illustrated in U.S. Pat. No. 3,636,716 issued Jan. 25, 1972 to Castellanos.
Yet another type of compliant tower, the "hybrid tower", is disclosed in U.S. Pat. No. 4,610,569 issued Sept. 9, 1986 and entitled "Hybrid Offshore Structure". Basically, a hybrid tower comprises a compliant upper section such as a guyed tower or a buoyant tower mounted on a rigid, bottom-founded lower section. As more fully described in the referenced patent application, the compliant upper section is permitted to pivot laterally a few degrees about a pivot point located at or near the upper end of the lower section in response to the applied environmental forces.
The compliant towers described above are generally designed for use as petroleum drilling and producing platforms. Another type of articulated offshore structure is the "single anchor leg mooring" or "SALM" which is used to transfer hydrocarbon products from the bottom of a body of water to a floating storage facility or tanker. Basically, a SALM is a riser pipe extending from a base located on the bottom of the body of water to a mooring buoy located on the surface of the body of water. A floating storage facility or tanker is moored to the mooring buoy and is allowed to "weathervane" about the buoy in response to environmental forces. Typically, a SALM is articulated or jointed at both the bottom and the top of the riser pipe. Hydrocarbon product flowlines extend from the base through the articulated joints, riser pipe, and mooring buoy to the floating storage facility or tanker. See, for example, the SALM illustrated in FIG. 1 of U.S. Pat. No. 4,337,970 issued July 6, 1982 to Gunderson.
Each of the articulated offshore structures described above requires the use of an articulated joint or pivot which will permit the desired lateral pivoting movement. Typically, the articulated joint or pivot must also be capable of transmitting large vertical loads between adjacent sections of the structure. Generally, the articulated joint or pivot is located near the bottom of the structure; however, as noted above, a SALM is typically articulated or jointed at both the bottom and the top of the riser pipe.
As an added complication, articulated offshore structures typically require use of a means to transmit torsional loads between adjacent sections of the structure. Offshore structures are seldom, if ever, perfectly symmetrical. Wind, waves, and ocean currents impinging on an asymmetrical structure create uneven forces which tend to twist the structure about its vertical axis. These twisting forces must be transmitted to and resisted by the foundation of the structure in order to prevent damage to or destruction of the flow lines, well conductors, and other components of the structure.
Heretofore, a variety of mechanical devices, such as universal joints and ball joints, have been proposed for use as articulated joints or pivots in articulated offshore structures. Use of a universal joint has the benefit of combining the articulated joint or pivot and the torsion means. See, for example, FIG. 10 of U.S. Pat. No. 3,626,701 issued Dec. 14, 1971 to Laffont. Use of a ball joint as the pivot requires use of a separate torsion means. See, for example, the hinge disclosed in U.S. Pat. No. 3,735,597 issued May 29, 1973 to Guy. Other types of mechanical pivots have also been proposed for use in articulated offshore structures. See, for example, U.S. Pat. No. 3,636,716 issued Jan. 25, 1972 to Castellanos and U.S. Pat. No. 4,231,632 issued Nov. 4, 1980 to Tuson.
The mechanical devices previously proposed for use as articulated joints or pivots in articulated offshore structures typically incorporate moving parts which are subject to wear and must be maintained over the life of the structure. In deep waters, maintenance of such devices is, at best, extremely difficult and expensive. Repair or replacement of such devices may be a practical impossibility. For large structures, or structures in deep waters, the loads which must be transmitted by such devices are so great that commonly used mechanical joints or pivots are impractical. Further, use of a mechanical joint or pivot typically requires that loads be transmitted through a single point which has no redundancy.
Accordingly, the need exists for a pivot or joint suitable for use in an articulated offshore structure which has no moving parts, is capable of transmitting large loads, is capable of providing redundancy, and requires little or no maintenance over the life of the structure.